Papers by Author: Xin Lin

Abstract: To improve the wear properties of AZ91D magnesium alloys, a AlFeCuCoNiCrTi_1.5 high entropy alloy (HEA) coating was fabricated on AZ91D magnesium alloys by laser cladding using mixed powders of Al, Fe, Cu, Co, Ni, Cr, and Ti. The microstructure of the HEA coating was characterized by OM, SEM, and XRD. The wear resistance of the HEA coating was evaluated under dry sliding wear test condition at room temperature. The results show that the HEA coating mainly consists of a simple BCC solid solution phase. The HEA coating exhibits excellent wear resistance. The main wear mechanisms of the HEA coating and the AZ91D substrate were different, the former dominated by oxidative abrasive wear and the latter suffered from both adhesive and abrasive wear.

Abstract: To improve the wear properties of AZ91D magnesium alloys, a Cu_0.9NiAlCoCrFe high entropy alloy (HEA) coating was fabricated on AZ91D magnesium alloys by laser cladding using prealloyed Cu_0.9NiAlCoCrFe powders. The microstructure of the HEA coating was characterized by OM, SEM and XRD. The wear resistance of the HEA coating was evaluated under dry sliding wear test condition at room temperature. The results show that the HEA coating mainly consists of a simple BCC solid solution phase. The HEA coating exhibits excellent wear resistance. The main wear mechanisms of the HEA coating and the AZ91D substrate were different, the former dominated by oxidative abrasive wear and the latter suffered from both adhesive and abrasive wear.

Abstract: To improve the wear property of 00Cr13Ni4Mo hydro turbine blade stainless steel, Ni-based composite coatings were fabricated on 00Cr13Ni4Mo stainless steel by laser cladding using mixed powders of Ni60, WC and TiN. The microstructure of the coatings was characterized by XRD and SEM techniques. The wear resistance of the coatings was evaluated under dry sliding wear condition at room temperature. The results show that the coatings mainly consist of Ni-based solid solution, WC and TiN phases. The coatings exhibit excellent wear resistance due to its high hardness of WC and TiN phases. The main wear mechanisms of the coatings and the 00Cr13Ni4Mo sample are different, the former is abrasive wear and the latter is adhesive wear.

Abstract: Iridium (Ir) could be taken as high temperature protective coating for the refractory metals. Ir coating was deposited on the surface of molybdenum (Mo) substrate by double glow plasma. Thermal stability of the coating was investigated at 1400°C for 90 min, while Ar gas was inputted to hold the vacuum pressure. The microstructure of the surface and interface of the Ir coating were observed by SEM and TEM. The phase transition of the coating was determined by X-ray diffraction. Many micropores and microbubbles appeared in the surface of the as-heat treated coating. The interfacial reaction between the Mo substrate and Ir coating occurred during heat treatment, and Ir21.5Mo8.5 phase was formed at 1400°C. The experimental results indicated that the integrity of the Ir coating was not degraded after heat treatment.

Abstract: A modified cellular automaton model for describing the dendritic solidification of pure substance was developed. Instead of using the high mesh-induced anisotropy capture rules, such as Von Neumann’s and Moore’s method, a new capture rule---random zigzag method was developed, which greatly reduced the mesh-induced anisotropy in crystallographic orientation. The calculation method for the solid/liquid interface curvature was also improved. The effect of interfacial energy anisotropy on the dendritic growth behavior was analyzed.

Abstract: Coarse aluminum surfaces were prepared by etching method, the wettability of NH4Cl -70wt%H2O solution on the surfaces was investigated, and then the substrates were undercooled and immerged into NH4Cl-70wt%H2O solution to trigger nucleation of NH4Cl crystals. Experimental results indicate that the aluminum surface with step-like micro-morphology shows a transformation from strong wettability to non-wettability during its exposure in the atmosphere, in the same time, nucleation density on the strongly wetting surface is significantly higher than that of the non-wetting surface. A hypothesis is proposed to explain the relationship between the wettability and the nucleation density: the capillary effect helps the solution infiltrate into the structures of the surface with strong wettability and drive the air out of the surface structures, so nucleation is effectively promoted by the sharp corners and hollows of the coarse surface; conversely, the air bubbles trapped in the non-wetting surface structures decrease the number of the effective nucleation sites, and leads to a higher nucleation energy barrier, thus the nucleation density is decreased.

Abstract: A two dimensions (2D) multiphase solidification model is used to study the liquid core solidification in the influence of deformation during soft reduction of continuous casting (CC). The transient transport equations (mass, momentum and enthalpy) for each phase of a thin steel slab CC are solved. Four different cases including of density-temperature function and deformation reduction factor on this CC are simulated. The solidification ending point position of liquid core, temperature, velocity and fracture of liquid and solid phases are compared. Understandings to the deformation and liquid core formation mechanism on soft reduction solidification process of CC are improved.

Abstract: A modified cellular automaton model was proposed to simulate the dendrite growth of alloy. Different from previous models, this model used neither an analytical equation(such as KGT model) nor an interface solute gradient equation to solve the velocity of solid-liquid interface, but used the interface solute and energy conservation and thermodynamic equilibrium condition to describe the solid/liquid interface growth kinetics process. In present model, once the temperature field and solute field were solved by finite different method in the entire domain, the material thermodynamic properties can be substituted into four algebraic equations to easily determine the variation of solid fraction, interface temperature and solute concentration, instead of calculating interface moving velocity. As a result, the complexity of the calculation can be largely reduced. The simulated dendrite growth was in a good agreement with the Lipton–Glicksman–Kurz (LGK) model for free dendritic growth in undercooled melts.

Abstract: Laser rapid forming (LRF) is introduced as a novel fabrication process for Ni-based superalloy Rene88DT. The effect of heat treatment parameters of quenching rate and aging time on size and distribution of γ′ precipitation was investigated. The heat treatment parameters were first determined by DSC, and then optimized based on the examination of the microstructure and mechanical properties of heat treated LRF Rene88DT. The experimental results show that, the precipitation of γ′ is inhomogeneously distributed as a result of uneven heat-cycle during LRF in as-deposited Rene88DT, resulting in low mechanical properties. After being heat treated at 1165°C, 2h/AC + 760°C,28h/AC, γ ′ precipitation are homogeneously distributed with the size of 40~60nm, and the tensile strength of heat treated sample shows an increase of 400MPa as compared to that for as-deposited. The yield strength is close to that of the PM+HIP standard.

Abstract: Ti-Ni based functionally graded alloy is a kind of the promising material, which has potential to be used in aero engines. Using laser rapid forming, a Ti-Ni graded alloy with a continuous compositional gradient from pure Ti to Ti-50wt.%Ni were fabricated. On comparison with the graded alloy, a series of homogenous deposits with the typical composition between pure Ti to Ti-50wt.%Ni were also laser rapid formed. The phase evolution along the compositional gradient direction in the graded alloy is: α+β→β+Ti2Ni →(TiNi +Ti2Ni)+ TiNi; and the phase evolution in the corresponding compositional homogeneous deposit is: α+β→ β+(β+Ti2Ni)→ β+Ti2Ni+(β+Ti2Ni)→ (TiNi+Ti2Ni)+TiNi. The phase transformation and microstructural evolution along the compositional gradient were analyzed by using the microstructure selection map.